During the first two years of this award, it was demonstrated that P selectin (CD62P) mediates migration of pioneer T lymphocytes into the central nervous system (CNS). These cells are called pioneer cells because they can enter the brain prior to the development of inflammation, and their function is to perform immune surveillance or to initiate inflammation. Interestingly, CD62 is expressed by meningeal vessels, pia-arachnoid, and choroid plexus epithelium, but not by parenchymal vessels. These findings suggested that normal immune surveillance of the brain is primarily directed to the meningeal-cerebrospinal fluid space as opposed to parenchymal perivascular regions. In order to study the role that CD62P-dependent T lymphocyte migration plays in autoimmune disease, the new goal addressed by this proposal is to develop an adoptive transfer model of mouse experimental autoimmune encephalomyelitis (EAE) that does not require any manipulations such as irradiation or adjuvants that can acutely alter expression of CD62P. Preliminary data demonstrate that (B10.PLxC57BL6/J) Fl mice develop severe adoptive transfer EAE without any prior activation or compromise of the host's immune system. Control B10.PL or (B10.PLxSJUJ)Fl mice either develop mild disease or have no clinical impairment at all despite the presence of meningeal inflammatory infiltrates. Based on these results, it is hypothesized that normal homing of T lymphocytes to the meningeal compartment may protect the host from developing severe forms of brain autoimmune diseases such as multiple sclerosis. The aims of this proposal are to further characterize the early inflammatory pathology in (B10.PLxC57BL6/J)F1, (B10. PLxSJUJ) Fl, and B10.PL mice, analyze the genetic complexity of this disease phenotype by classical genetic methods, and to determine the role that CD62P may have in these disease phenotypes by using knockout mice. The initial goal is to determine the critical time at which commitment to a severe disease phenotype is made in (B10.PLxC57BL6/J)Fl mice. The long-term goal is to identify methods that permit normal immune surveillance of the brain but inhibit the initiation of autoimmune disease.